Among semiconductor devices, wide bandgap semiconductor devices, featuring advantages of superior saturation electron velocity, pressure field and heat dissipation coefficient, have attracted many industrialists or research organizations for further development. Currently, some most commonly used wide bandgap semiconductor devices are gallium nitride (GaN) and silicon carbide (SiC) semiconductor devices.
Taking a high electron mobility transistor (HEMT) for example, it is characterized by having a high frequency, a high breakdown voltage and a low loss. Common HEMTs include pure enhancement-mode high electron mobility transistors (pure E-mode HEMTs), transistors having a embedded clamping diode design, and cascoded low-voltage metal-oxide-semiconductor field-effect-transistors (LV-MOSFETs).
Among the above, a pure E-mode HEMT has a difficult manufacturing process, and includes a gate insulation layer having a weak structure that thus disfavors applications. In addition to a drawback of being controlled by only pulse width modulation (PWM), a transistor having an embedded clamping diode design further suffers from issues of being in a normally on state when activated and requiring precise gate driver designs. Further, a cascoded LV-MOSFET has issues of having high packaging costs, a high conduction impedance and a slow switching speed, and is not considered as an optimal solution for E-mode HEMTs. A structure of a cascoded LV-MOSFET may be referred from below.
The U.S. Pat. No. 8,624,662 B2 discloses an electronic device including a depletion-mode transistor, an enhancement-mode transistor, and a single package. The single package encases the depletion-mode transistor and the enhancement-mode transistor. A source electrode of the depletion-mode transistor is electrically connected to a drain electrode of the enhancement-mode transistor, a drain electrode of the depletion-mode transistor is electrically connected to a drain lead of the single package, a gate electrode of the enhancement-mode transistor is electrically connected to a gate lead of the single package, a gate electrode of the depletion-mode transistor is electrically connected to an additional lead of the single package, and a source electrode of the enhancement-mode transistor is electrically connected to a conductive structural portion of the single package. The gate electrode of the depletion-mode transistor is not electrically connected to every electrode of every transistor encased in the single package.
Alternatively, the U.S. Pat. No. 8,084,783 B2 discloses an enhancement-mode GaN FET device including a main GaN FET and a switching element. The switching element and the main GaN FET are connected in a cascoded configuration. The switching device includes an FET connected in parallel to a diode switching structure. Wherein, the main GaN FET cascoded with the switching device operates as an enhancement-mode GaN FET device, and the GaN FET is monolithically integrated into the same substrate as the FET and the diode switching structure of the switching device.
A conventional wide bandgap power device such as an SiC JFET and GaN HEMT device, although featuring advantages of having a high switching speed, a high voltage tolerance and a low conduction impedance, frequently encounters a bottleneck when manufactured as an enforcement-mode device. Further, in actual applications of a power device, due to an extremely large power carried by the power device, the circuit may become short-circuited in the event of a control issue when used as a normally on device (a depletion-mode device). Thus, an enormous current caused may pass through to not only damage the circuit but also potentially threatens the safety of the operator.
For example, taking a GaN HEMT for instance, due to its 2-dimensional electron gas (2DEG) property, the manufacturing of depletion-mode devices is much more low-cost and simpler than the manufacturing of enhancement-mode devices. Thus, manufacturers have introduced conventional technologies of silicon devices to change the normally on characteristic of a GaN HEMT device using a cascoded configuration, so that such device may be applied as a composite normally off device. While providing the characteristic of having a high breakdown voltage as a wide bandgap transistor, such types of devices are capable of simultaneously driving the overall device by a driving approach as a conventional siliceous MOSFET. However, as carried devices may still be siliceous devices, the switching speed may not be quite equal to that of pure wide bandgap devices. Further, due to the cascoded configuration, the conduction resistance of crystals is also increased, such that the advantages of wide bandgap devices cannot be effectively exercised.
In conclusion, known from the examples of the above enhancement-mode HEMTs, there is a need for a solution for improving technologies of wide bandgap semiconductor devices.